This invention generally relates to ultrasound imaging for the purpose of medical diagnosis. In particular, the invention relates to methods for imaging tissue and blood flow by detecting ultrasonic echoes reflected from a scanned region of interest in a human body.
Conventional ultrasound scanners are capable of operating in different imaging modes. In the B mode, two-dimensional images can be generated in which the brightness of each display pixel is derived from the value or amplitude of a respective acoustic data sample representing the echo signal returned from a respective focal position within a scan region.
In the B-mode imaging, an ultrasound transducer array is activated to transmit beams focused at respective focal positions in a scan plane. After each transmit firing, the echo signals detected by the transducer array elements are fed to respective receive channels of a receiver beamformer, which converts the analog signals to digital signals, imparts the proper receive focus time delays and sums the time-delayed digital signals. For each transmit firing, the resulting vector of raw acoustic data samples represents the total ultrasonic energy reflected from a succession of ranges along a receive beam direction. Alternatively, in multiline acquisition two or more receive beams can be acquired following each transmit firing.
In conventional B-mode imaging, each vector of raw acoustic data samples is envelope detected and the resulting acoustic data is compressed (e.g., using a logarithmic compression curve). The compressed acoustic data is output to a scan converter, which transforms the acoustic data format into a video data format suitable for display on a monitor having a conventional array of rows and columns of pixels. This video data is referred herein as xe2x80x9craw pixel intensity dataxe2x80x9d. The frames of raw pixel intensity data are mapped to a gray scale for video display. Each gray-scale image frame, hereinafter referred to as xe2x80x9cgray-scale pixel intensity dataxe2x80x9d, is then sent to the video monitor for display.
A conventional ultrasound imaging system typically employs a variety of gray maps, which are simple transfer functions of raw pixel intensity data to display gray-scale values. Multiple gray maps are supported so that different maps may be used depending on the range of pixel intensities. For example, if a given application tends to generate mainly low raw pixel intensities, then a gray map which dedicates more gray-scale values to low raw pixel intensity values is desired since it improves the contrast across this range. Therefore, it is typical to default to a different gray map depending on the application. However, this is not always effective since the user can scan any anatomy in any application, acoustic data varies from patient to patient, and the raw pixel intensity values depend on other system settings such as dynamic range. Due to these factors, the gray maps tend to be conservative with respect to how many gray-scale values are dedicated to the anticipated primary pixel intensity range.
A xe2x80x9cone-touchxe2x80x9d automatic tissue optimization (ATO) method is known which allows the system user to adjust the contrast by pressing a so-called ATO button on an operator interface. When the user has positioned the probe over the anatomy of interest, depressing an ATO button triggers the host computer inside the ultrasound imaging system to retrieve the current frame of raw pixel intensity data, analyze its pixel intensity histogram within a user-specified region of interest (ROI), and then automatically scale and/or shift the gray mapping (i.e., raw pixel intensity to gray-scale pixel intensity mapping) such that pre-defined xe2x80x9coptimalxe2x80x9d upper and lower gray-scale levels map to some upper and lower bounds of the pixel intensity histogram respectively. The ultimate goal is to more fully utilize the available gray-scale levels (256 levels for an 8-bit display system) to display the pixel intensity data, thereby improving the display tissue contrast.
In the one-touch ATO approach, however, if the probe or ROI is moved to another location, the user is required to press the ATO button again to re-optimize the gray mapping based on the new tissue data. A more fully automated version of this feature is desirable because during a clinical exam, the sonographer often needs to move the probe around a lot to find or study multiple anatomical features, and in many clinical applications such as vascular and surgical applications, both of the sonographer""s hands are already busy or sterilized.
The present invention is a method and an apparatus for optimizing operating parameters in an ultrasound imaging system in response to the occurrence of predetermined changes in the pixel intensity histogram of successive image frames. In the course of re-optimization, mapping, compression, scaling or beamforming parameters can be adjusted based on pixel intensity histogram characteristics determined by the computer.
The method in accordance with the preferred embodiment comprises the steps of monitoring changes in the pixel intensity histogram of successive image frames, which may be indicative of probe movements, and when appropriate, automatically triggering re-optimization of the operating parameters. The assumptions are as follows: (1) as long as the pixel intensity histogram is changing (the ultrasound probe is moving), the sonographer is doing general looking around; and (2) when the pixel intensity histogram has evolved into a new, stable form for a preset amount of time (the probe is held still again), the sonographer has found something interesting to look at. In response to satisfaction of these two conditions, the relevant operating parameters are re-optimized. In accordance with one preferred embodiment, the compression curve and/or the gray mapping are automatically optimized (e.g., set to values which optimize contrast in the displayed image). In accordance with other preferred embodiments, the beamforming parameters or the scaling parameters can be automatically adjusted to display an image in a zoom mode. In accordance with the preferred embodiments, the pixel intensity histogram analysis and the re-optimization of the operating parameters in dependence on the histogram analysis results are performed by the host computer incorporated in the ultrasound imaging system.
It should be noted that in practice, probe motion may not always cause large changes in the pixel intensity histogram, especially if the probe remains in good contact with the skin surface and the underlying tissue characteristics happen to be quite uniform. If the pixel intensity histogram has changed a lot, however, chances are that significant probe motion has occurred. Thus, the triggering mechanism for image optimization is based on pixel intensity histogram changes and not probe motion per se. In the case of the data compression curve and gray mapping, as long as the pixel intensity histogram remains relatively unchanged, there is no need for re-optimization regardless of probe motion.
The invention makes ultrasound imaging systems easier to use. Examination times will be shortened due to less downtime spent optimizing the mapping parameters. The invention will also facilitate standardization or reproduciblity of exams done by different sonographers. Finally, the invention allows xe2x80x9chands freexe2x80x9d scanning during surgical, vascular and other applications where both hands are already busy or sterile.